1. Field of the Invention
This invention relates to blood separating compositions and, more particularly, this invention relates to a radiation-sterilizable silica-silicone fluid gel useful for partitioning the serum and clot portions of a blood sample.
2. Brief Description of the Prior Art
The use of thixotropic gels as partitioning materials in blood collection and separating devices is well known. In such devices, a thixotropic gel having a density intermediate that of the serum and clot portions of a blood sample is disposed in a fluid collection device such that, upon centrifugation, the serum and clot portions separate with the gel disposed intermediate the portions to provide a chemical and physical barrier therebetween, thus facilitating separation of the respective portions for analysis. A preferred form of blood collection device is described in Murty U.S. Pat. No. 4,180,465, the disclosure of which is hereby incorporated by reference.
One well known type of thixotropic gel material is a silica-silicone fluid gel formed by the reaction between silicone fluid, a filler material (such as silica particles) and a thixotropic property-imparting amount of a network former. (Silicone fluids used to prepare such gels are conventionally characterized as "silicone oils", and the terms "silicone fluid" and "silicone oil", as used herein, are interchangeable.)
Hertl U.S. Pat. No. 3,977,982 and Zine, Jr. U.S. Pat. No. 4,049,692, the disclosures of which are hereby incorporated by reference, describe typical silica-silicone fluid gels and methods of preparing the same.
It is desirable, however, that a thixotropic gel used in a blood collection device be sterile in order to prevent backflow contamination to the patient. A preferred method of sterilizing a blooc collection device and gel barrier disposed therein is the use of .gamma.-radiation, or another type of electron capture radiation. Prior silica-silicone fluid gel serum separators, however, when sterilized by radiation, harden to the point at which they are no longer functional, exhibiting viscosities in the 3,000,000 to 5,000,000 centipoise (cp) range.
Other types of thixotropic gel barrier materials which are functional after sterilization by radiation have been developed. One such material is described in Semersky U.S. Pat. No. 4,235,725, the disclosure of which is hereby incorporated by reference. The Semersky patent describes a blood collecting and separating device in which a thixotropic gel separating material comprises a mixture of liquid polybutadiene and an inorganic, inert filler which is adapted to form at rest a sealing barrier between separated blood phases. The device and gel of the Semersky patent may be subjected to sterilizing radiation to render the device and gel free of backflow contamination, allegedly without degradation of the physical properties of the gel.
While polybutadiene and other polymeric gel materials may be sterilized by exposure to radiation with only limited degradation of the gel properties, such materials tend to harden with time and thus have limited shelf lives. Silica-silicone fluid gel barrier materials, on the other hand, do not harden to a substantial degree with time, and thus exhibit extremely long shelf lives. Further, silica-silicone fluid gels tend to provide a superior chemical and physical barrier (i.e. better resistance to leakage or slippage) in a collection tube than do other polymeric barrier materials.
Thus, the need for a sterilizable silica-silicone fluid gel material useful as a blood separating composition is apparent.